The Rules of Forgetfulness (Sahw) in Prayer
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 50
وَ رَوَى مُحَمَّدُ بْنُ مُسْلِمٍ عَنْ أَبِي عَبْدِ اَللَّهِ عَلَيْهِ اَلسَّلاَمُ أَنَّهُ قَالَ: «إِنْ شَكَّ اَلرَّجُلُ بَعْدَ مَا صَلَّى فَلَمْ يَدْرِ أَ ثَلاَثاً صَلَّى أَمْ أَرْبَعاً وَ كَانَ يَقِينُهُ حِينَ اِنْصَرَفَ أَنَّهُ كَانَ قَدْ أَتَمَّ لَمْ يُعِدِ اَلصَّلاَةَ وَ كَانَ حِينَ اِنْصَرَفَ أَقْرَبَ إِلَى اَلْحَقِّ مِنْهُ بَعْدَ ذَلِكَ.
Muhammad ibn Muslim narrates from Imam Jafar ibn Muhammad Al-Sadiq عليه السلام: "If a man is uncertain after he has prayed and does not know whether he prayed three or four rak'ahs, and when he finishes, he is certain that he has completed it, he does not need to repeat the prayer. At the time of his departure (after the prayer), his certainty is closer to the truth than it was afterward."
- Ṣaḥīḥ al-Faqīh, p. 47